Abstract: It was shown that the -SH group in molecules of captopril (CTPR) exhibits the property of reduction of Fe(Ⅲ) to Fe(Ⅱ), and that the color of complex formed between Fe(Ⅲ) and tiron is extincted by the presence of CTPR. Conditions for color reaction between Fe(Ⅲ) and tiron were taken as follows: in a 25 mL volumetric flask, 2.20 mL of 0.250 0 g·L-1 Fe(Ⅲ) solution, 0.6 mL of 6.25 g·L-1 tiron solution and 10.0 mL of Clark-Lubs buffer solution of pH 3.0 were added and water was added to make up the volume to 25 mL. The reaction mixture was heated in water bath at 80 ℃ for 20 min for completion of the color reaction. After cooling absorbance (A1) of the solution is measured at its absorption maximum of 670 nm. Standard solutions of CTPR with various concentrations were added to solutions of the color reaction system described above and operated in the same way. Values of absorbance (A2) were measured, and linear relationship between values of ΔA(A1-A2) and mass concentration of CTPR was kept in the range of 0.012-0.036 g·L-1. The above procedure was applied to determination of CTPR in its tablets, giving results in consistency with the values found by the method of pharmacopoeia.